OxygenOS Open Beta 3 (7.1.1) is now available for the OnePlus 3T; the same update is also available to download for the older OnePlus 3 as Open Beta 12 (7.1.1). The company's release notes for the beta are identical for both handsets:

Upgraded to Android 7.1.1

New Shot on OnePlus wallpaper option

Gallery Improvements

New photo editor with watermark and photo straightening options

Newly added media will be highlighted in Photos section

Image location will be displayed on grid view

Media can be grouped by month and year

Folders and individual media can be hidden

General bug fixes

There are also a couple of known issues to be aware of before installing the beta on your device. OnePlus warns of "compatibility issues with certain third party apps", although it doesn't offer any examples of which apps might be affected. The company also says that Android Pay may not work on public transit services operated by Transport for London, but adds that "this can be resolved by updating the app via [the Google] Play store".